Hi everyone, I had the same issues that have been described here a number of times...
I have a Samsung NP540u3c which comes with the centrino advanced 6235n. After trying many driver updates and settings tweaks with little to no success, I decided to contact samsung to repair the laptop - but I didn't just tell them to see what's wrong and try to fix it, but I specifically asked them if it would be possible to swap the intel wifi card for another, from a different manufacturer. I also mentioned this thread as a reference that I'm not the only one with this issue or that it isn't common with this card (unfortunately).
They said ok 
and offered to swap it for an atheros card, with the repair covered by my warranty... So I sent them the laptop, and it took them two weeks to call me. When they did, they told me a peculiar thing - that they tried to swap the wifi card, but the atheros card had the same issues that the 6235 had! loss of signal, dropping connection and unable to reconnect, not finding any networks nearby, etc.
So they tried to find the real problem and were able to - they said the cables that connect the wifi to motherboard were in fact defective, and when they changed them for new ones, even the 6235N worked perfectly!
So they decided to leave the intel card in, tested it for another week and then sent the PC back to me. It's been a week since then and I tested it on a number of networks, with downloads and streaming videos, and I'm more than happy to report that the connectivity issues seem to be gone for good! I don't have the latest 15.8. drivers installed, just 15.5. or whatever came originally from samsung, and I still don't dare to update them to not mess anything up (very superstitious after the issues I had - y'all know what I'm talking about), but they seems to work well and I've experienced no hiccups at all.
One more thing to mention is that I haven't tried a "n" connection yet (I have none available), but considering that there are no problems on any of the "g" networks where they had been before, I think the "n" portion was fixed with new cables as well.
So the above might be a solution for all that still have a valid warranty and don't feel like voiding it (by swapping the wifi by themselves) - the guys at local samsung support were very helpful and were ready to swap the card, no questions asked - but in the end they even didn't have to...
Hope this helps...